Four essential conditions govern good desulfurization of molten iron or steel by a slag phase. These are: (1) high basicity or V ratio, (2) high temperature, (3) low oxygen potential, and (4) high slag-metal emulsion rate. The foregoing essential conditions were first recognized and applied with success by Rene Perrin of Ugine Aciers, France around 1937. Perrin prefused an artificial slag in a separate furnace, the slag composition being about 50% CaO and 50% Al.sub.2 O.sub.3. After thoroughly deoxidizing slag and steel in an electric arc furnace prior to tapping, Perrin poured the hot, liquid and prefused artificial slag onto the bottom of the ladle and immediately tapped the steel from the furnace into it. This tapping operation routinely brought the sulfur content from 0.025% down to 0.005% or less in the ladle sample, an 80% drop. This is equivalent to the results obtained today with lengthy post-tapping injection procedures.
While the method proposed by Perrin was exceedingly effective to lower sulfur contents in steel, it will be readily appreciated that predeoxidation of the steel and the slag in the furnace and separate prefusion of artificial slags are procedures too expensive and time-consuming for commercial applications.
In U.S. Pat. No. 4,142,887 a ladle desulfurization composition and method are disclosed in which molten steel to be desulfurized is exposed to a mixture of particulate metallic aluminum, fluorspar and lime to deoxidize and desulfurize the metal and form a fluxed slag. The desulfurizing composition described in the aforesaid patent accomplishes effective desulfurization of molten steel without predeoxidation and separate prefusion of the slag; but the composition extracts intrinsic heat from the molten steel and rapidly cools the molten steel. Accordingly, higher temperatures are required in the molten steel to supply the intrinsic heat required and satisfy the second essential condition described above.